Installing chair rail over uneven sheetrock

I'm in the process of painting my home office and installing chair rail. The painting is done and having trouble with the chair rail. It appears that a couple of areas of the wall are a bit concave. When I put the chair rail up, there are 2 sections where the gap between the chair rail and the sheetrock is up to 3/8" at it's worst, and the gap is about 2 feet long.

My question is how do I fill this gap so that I can caulk and paint the chair rail (and caulk)? Please keep in mind that I'm done painting the walls, and since this is not a heavily traveled area, mudding the entire wall (or general portion outside of the rail) is out of the question.

I would think about fixing the wall. (Painting isn't THAT much work.) if you decide against it I would use some wood strips to fill the longer/wider gaps. Just caulking the wider bits is likely to look bad. At least use some caulking backer so it won't sag terribly.

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If I could only remember to THINK about what I was doing before I did it.

Thanks for the replies! At this point, I'm ready to be done with everything. I'm considering a couple of options. One, return the wood chair rail in favor of the more flexible PVC version to bend with the wall. Two, do something similar to what Leah suggested and use some styrofoam to fill the gaps behind the rail. Hopefully, this will get me to a point where I can caulk the heck out of it and get the rail painted.

Thanks for the replies! At this point, I'm ready to be done with everything. I'm considering a couple of options. One, return the wood chair rail in favor of the more flexible PVC version to bend with the wall. Two, do something similar to what Leah suggested and use some styrofoam to fill the gaps behind the rail. Hopefully, this will get me to a point where I can caulk the heck out of it and get the rail painted.

DONT USE THE PVC. You will hate yourself later. You can kerf the wood from the back to make it easier to bend into shape.

__________________
If I could only remember to THINK about what I was doing before I did it.

I have a similar situation in my pantry where the hot water pipe from the water heater on the other side of the wall goes down in the wall and apparently was situated past the face of the studs a bit. So the idiot drywall guy just bent the drywall over the pipe. As such my baseboard has a pretty decent bend in it. The wall is only about 5 feet long and I just pushed the baseboard against the wall and finished nailed it into studs in several places.

The thought that came to my mind is the possibility of using screws. Counter sink the screws and when tightening them, just split the difference as far as the gap goes. Just would want to be sure to hit the studs with the screws. This way the caulked gap would be smaller and hopefully the curve in the rail won't stand out. But that's me. Others mileage may vary.

Similar Problem - Chair rail on uneven wall

I have a 30 year old wall with bowed drywall between 16 inclh studs. The bow is almost 1/4 inch and too much to caulk. My only idea is to use a two piece chair rail. First, mount the back board using mollybolts to pull the drywall to the chair rail backing board. The top piece of the chair rail will cover the molly bolt heads.

Any better ideas? I don't want to use that big a chair rail. I don't think I can conseal the molly bolts in a single piece of chair rail.

Take the chair rail down at the concaved area, take a reciprocating saw or circular saw and cut the studs about an 1 1/2 inches in where the chair will cover the cut. Have someone go on the others side of the wall and push the studs one at a time toward the curve in the wall. Have a couple of tapered shims ready and push then into the cut. Let the other guy let off pushing and see if that moved the stud into the curve, if not enough, let the other fellow push again and keep doing that until the wall is straight. Cut the shims off and put the chair back over the cuts and there ya go.